Stress corrosion crack inhibitors



United States Patent 3,378,411 STRESS CORROSION CRACK INHIBETORS Charles R. Bergen, Franklin, Wis, assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis. N0 Drawing. Filed Nov. 27, 1964, Ser. No. 414,467 Claims. (Cl. 148--6.35)

ABSTRACT OF THE DISCLOSURE A process for inhibiting stress corrosion cracking in austenitic stainless steels subjected to chloride environments wherein the surface of the steel is provided with an oxide film containing ions of silver, lead and/or cobalt in amounts of less than 2 ,ugm. per square centimeter.

This invention relates generally to stress corrosion cracking. More specifically, this invention is concerned with stress corrosion crack inhibitors and methods for inhibiting stress corrosion cracking, particularly in austenitic stainless steel.

Stress corrosion cracking has been defined as failure by cracking due to the combined action of corrosive material and stress, the stress being either external (applied) or internal (residual). Generally, the cracking may be either intergranular or transgranular, depending upon the stressed metal and the corrosive material.

Not all metals susceptible to stress corrosion cracking are uniformly affected by any particular corrodant. For example, carbon steels are most susceptible to stress corrosion cracking in nitrate environments, and copper alloys are most affected by ammonia, while austenitic stainless steels are most susceptible to stress corrosion cracking in chloride environments.

One of the most troublesome areas of stress corrosion cracking has been that of austenit-ic stainless steels in contact with chloride environments. Some chloride solutions, such as alkaline or alkaline-earth chlorides, are so aggressive when heated that they will cause highly stressed austenitic stainless steels to crack in extremely short periods of time, which may be less than about 30 minutes. Extensively cold-worked or as-drawn parts are especially susceptible because of the high degree of internal stresses. However, even annealed parts will fail in relative short periods of time under extreme conditions and external stresses. On the other hand, completely unstressed austenitic stainless steel would be excellent for use in contact with chloride solutions because of its resistance to ordinary corrosion efiiects.

The ferritic and martensitic stainless steels are also subject to stress corrosion cracking to a more limited extent. However, the problem is not so serious with these stainless steels because the martensitic stainless steels are quite uncommon, and the ferritic stainless steels cannot be used in chloride environments because they will be badly pitted and corroded.

Since the mechanism of stress corrosion cracking has not yet been established, the prior art has shown very little that can be done to prevent it. Some techniques have been developed, although they are not highly successful or desirable.

One method to eliminate or reduce stress corrosion cracking has been to design the structure so that no stresses (external or internal) are present. For example, austenitic stainless steels have an abnormally low carbon content, or containing carbon stabilizing additives, can be given a low temperature stress relief for periods of from 1 to 3 days to greatly reduce internal stresses. Subsequently, the applied stress can be reduced by overdesigning" the structure where the steel is used. This technique, however, cannot always be practiced because the low temperature anneal is not desirable for austenitic stainless steels having a more normal carbon content because such an anneal will cause carbide precipitation, making the steel more susceptible to ordinary corrosion. Furthermore, the cost of this technique would be excessive because of the quantity of steel used in over-designing the structure.

Another method that has been successful to a limited extent has been to add phosphate ions to the corrodant. It is believed that the phosphate ions tie up the corrosive chloride ions to reduce stress corrosion cracking. How ever, for unknown reasons, this method is not successful in situations where the steel is alternatively dried and rewetted by the corrodant.

The only known way to completely assure that stress corrosion cracking is not a problem is to employ structural materials which are not subject to stress corrosion cracking in the particular environment for which the structure is intended. Thus where structures are to be in contact with heated chloride solutions, the most common practice in the past has been to build the structure of high nickel alloys or steels plated with nickel or exotic metals such as platinum. Accordingly, an entire nonsusceptible surface is exposed to the corrodant. Although this does solve the problems of stress corrosion cracking, it also greatly adds to the cost of the structure for obvious reasons.

Thus, since austenitic stainless steel is quite resistant to the ordinary corrosive effects of chloride environments and comparatively inexpensive, there is a heartfelt need for methods to prevent stress corrosion cracking in the steel.

This invention is predicated upon my discovery that an oxide film containing silver, lead, or cobalt ions on the surface of an austenitic stainless steel is an excellent stress corrosion crack inhibitor when the steel is subjected to a chloride environment. Such an inhibitor is quite effective even in boiling, highly concentrated alkali chloride solutions wherein the steel is stressed to points Well beyond the yield strength.

Accordingly, it is a primary object of this invention to provide an inexpensive method for inhibiting stress corrosion cracking of austenitic stainless steels subjected to chloride environments.

It is another primary object of this invention to provide an oxide film, containing certain impurity cations therein, on the surface of austenitic stainless steel which will inhibit stress corrosion cracking in the steel.

These and other objects and advantages are fulfilled by this invention as will become apparent from a full understanding of the following detailed description.

As noted above, the stress corrosion crack inhibitor of this invention is an ordinary oxide surface film containing silver, lead and/or cobalt as a trace impurity. Practically any conceivable method for applying the oxide film and the impurity will sufiice. It should be understood, however, that neither the oxide film nor the impurity ions therein should provide a protective plating or coating. On the contrary, the exposed metal surface may be that of any ordinary austenitic stainless steel except that an extremely small amount of silver, lead and/or cobalt is present in the otherwise ordinary oxide surface film. Of course, if the impurity metal is deposited to such an extent that a complete protective coating or plating is provided, then obviously, this too would prevent stress corrosion cracking because an entirely new surface would be exposed to the corrodant which is not subject to stress corrosion cracking. However, the crux of this invention resides in the fact that inhibitor is not a protective coating, but an ordinary oxide film containing only a trace amount of chemically active impurities to prevent stress corrosion cracking. Generally, it can be said that a solid protective plating would at the very least have to amount to about 2 gm./cm. By this invention however, substantial increases in life of the steel can be obtained by using less than 2 gmjcmfi, and even less than about 1.5 ,ugm./cm. which is the limit of detectability. Thus it is quite clear that the mechanism of this inhibitor is chemical rather than a mere protective plating or coating.

In a preferred practice of the present invention, ions of silver, lead and/ or cobalt, or minute particles of these metals, are deposited onto the ordinary oxide surface film of an austenitic stainless steel. By an ordinary oxide surface film, I mean that the steel surface need not be especially conditioned to receive the metal, such as by polishing or removal of surface corrosion. In fact, to practice this invention it is necessary that an oxide film be present on the surface of the steel either before or after the silver, lead or cobalt is applied. Therefore, ordinary structural stainless steels which have been annealed in air should already have a sufiicient oxide surface film to satisfy the requirements of this invention.

A convenient method of applying silver or lead is by burnishing. That is, the metal is rubber onto the surface of the steel in much the same way as one would cover a piece of paper with pencil marks. By this method all that is needed is a solid piece of silver or lead which is then rubbed over the steel surface in a reasonably uniform manner so that most of, if not all, the steel surface has been covered. This should be followed by heavily burnishing the steel surface with paper, cardboard or the like to remove nonadherent deposits. The deposits which remain on the steel surface, although being only a trace amount, will be sufficient to provide enough cations of silver or lead, as the case may be, in the oxide film to greatly inhibit stress corrosion cracking. The quantity of impurity metals present however will be below the limits of senistivity of most ordinary means of analysis, which is below 1.5 ngm./cm.

In some instances, for example when the steel does not already have an oxide surface film, it may be desirable to apply the impurity metal directly onto the clear metal surface, and then allow ordinary corrosion processes to produce the oxide film. Such corroding processes are well known and need not be detailed here.

As may be expected, the burnishing method of application would not be highly satisfactory for applying cobalt because cobalt is so hard that very little of the metal would actually be deposited.

To test these stress corrosion crack inhibitors under the above described method of application, samples of type 304 stainless steel were tested without any metallic surface impurities and with silver and lead applied as described above. The coated and uncoated samples were given a uniform U-bend 'and placed in a boiling solution of concentrated calcium chloride. Table I below give's representative results of the tests.

TABLE I Average hours for ap- Coating on U-bent steel: pearance of cracks Uncoated 8 /3 Le'ad burnished 36 Silver burnished 1 No cracks in 112 hours.

Table I above suggests that the inhibiting properties of the lead containing film are far more limited than those containing silver. For reasons to be subsequently explained, this is as would be expected. Thus, the lead is capable of increasing the stress corrosion life about 27 hours or approximately 200%, while silver increases stress corrosion life at least 1000%.

Another method for applying the silver, lead or cobalt, which is perhaps more scientific, is to flash plate the metal onto the steel surface by electroplating techniques. Since a full protective plating is not necessary, the plating time should be kept extremely short so that only randomly distributed ions are deposited. Since there are various well known methods for plating silver, lead and cobalt, such techniques need not be detailed here. As a brief example however, it is well known that silver can be plated from an alkaline-silver cyanide solution. Therefore, if the austenitic stainless steel to be protected is made the cathode in a cell having an alkaline-silver cyanide electrolyte, with a cathode current density of say about 0.3 amps/cm. for a period of from 1 to 60 seconds, then sufiicient silver will be deposited to satisfy this invention.

Although the electrodeposition can be effected directly onto the oxide film in most cases, it may be more easily deposited onto a cleaned steel surface. If that is desired, the oxide film should be removed from the stainless steel surface by a suitable acid pickling solution before the metal ions are deposited by electroplating. Such scale and film removing processes are well known. Then after the metal ions have been deposited, the steel may be recorroded in a suitable oxidizing environment. For ex ample, the steel may be heated in air to a temperature of 1700 F. for a period of about two minutes or exposed to an aqueous caustic solution.

Although this method of application also greatly enhances stress corrosion life, no tests were made on samples by this method because of the difficulty in separating the protective barrier effects of possible continuous plate from the inhibitor effects of possible continuous oxide.

Other means of applying the impurity metal should be obvious. For example, the silver, lead and/0r cobalt may be directly alloyed with the steel. Then by reoxidation, sufficient cations will corrode to be incorporated in the oxide surface film.

Although the mechanism of stress corrosion cracking has not been established in the prior art, I have made some discoveries which do tend to explain this phenomenon and accordingly, explain the mechanics of my stress corrosion crack inhibitors. Specifically, it is well established that austenitic stainless steels subjected to chloride environments will contain chloride ions within the surface oxide film. I have found that these chloride ions will migrate through the oxide film in a tensile stress gradient so that the chloride ions are concentrated at points of highest tensile stress. This migration is driven by the free energy relationships of the oxide and chloride ions within the oxide film. That is, the thermodynamic properties of oxide and chloride are essentially identical except that the chloride ion is larger in mass and size. Accordingly, in a region of tensile stress the larger size of the chloride ion leads to reduced free energy when substituted for an oxide ion. Thus, to achieve thermodynamic equilibrium, or at least to progress in that direction, the chloride ions will migrate across the stressed surface concentrating in the region of highest tensile stress, and being the least concentrated in regions of compressive stress. In time, the surface regions of high tensile stress will consist primarily of a chloride surface film rather than an oxide film. At these locations, hydrolysis effects will cause release of chloride and hydrogen ions which will rapidly corrode the metal surface causing it to crack.

In the practice of my invention, the impurity metal cations are provided in the oxide surface film. These cations have a sufficiently stronger affinity for the chloride ions than for the oxide ions, and thus they will react with the migrating chloride ions to halt the migration and prevent concentration buildup of chloride ions at points of high tensile stress. It is improbable that sufficient energy would become available to dislodge the chloride ion so that migration would continue. Table II below shows the ratio 3 of chloride vs. oxide bond strength for the metal impurities.

TABLE II Bond strength Cation: (Cl/O) at C. Silver 20.6 Lead 1.66 Cobalt 1.33

From Table II it is seen that the bond strength of the silver chloride is over 20 times greater than the bond strength of the silver oxide, whereas the bond strength of the lead chloride is not twice that of the lead oxide. Accordingly, it would be expected that an oxide film containing silver would be a far better inhibitor than one containing lead. This was shown to be the case in Table I.

To aid in a fuller understanding of the invention, the following examples are presented as being typical. However, these examples are meant only to be illustrative of the invention herein described.

EXAMPLE I Two pieces of 304 stainless steel measuring 1 /2" x /8" x 0.060" were bent around a mandrel providing a 180 bend with an internal radius of 0.125 inch. Holes were drilled through the straight portions of the legs and No. 6-32 x /2" bolts were inserted. The legs were drawn down to leave only a 0.075 clearance. Lead was applied to the surface of the steel at the U-bend of one of the pieces by rubbing a lead sheet against the steel. The surface of the steel was then rubbed with a piece of paper to smooth the lead over the surface and remove any excess. Both U-bends were exposed to a boiling solution of CaCl saturated with sufficient CaCl to yield a boiling point of 132 C. The uncoated U-bend showed cracks in 6 hours and failed shortly thereafter. The inhibitor coated U- bend showed no signs of cracking in over 50 hours.

EXAMPLE 11 Two pieces of 304 stainless steel measuring 1 /2 x /8 x 0.060 were bent around a mandrel providing a 180 bend with a radius of 0.125 inch. Holes were drilled through the straight portions of the legs for the purpose of inserting No. 632 x- /2" bolts. One of the U-bends was coated with silver over the bent surfaces by the following electroplating procedure. The U-bend was made the oathode. The anode consisted of a length of carbon rod wrapped with cotton for /2" at one end to give a radius for the cotton of /3". After cleaning the specimen, the cotton anode was dipped in a silver cyanide plating solution and passed smoothly and quickly over the sides and curved portions of the U-bend once or twice to insure that a thin, but not necessarily continuous film of silver was applied. For the conditions herein described, about seven volts were applied. This technique provides approximately 1.5 ,ugms. of silver per cm.

Both of the U-bends were placed in a CaCl solution with sufificient CaCl present to produce a boiling point of 132 C. The unprotected specimen showed cracking after eight hours and failed shortly after. The inhibitor coated specimen did not show cracks for 120 hours at which time the test was terminated.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of inhibiting stress corrosion cracking in austenitic stainless steels subjected to chloride environments which comprises; cleaning the austenitic stainless steel surface with a suitable acid to remove any corrosion products therefrom, depositing ions selected from the group consisting of silver, lead and cobalt onto the cleaned steel surface in amounts of less than 2 ,ugm. per square centimeter, and corroding the steel surface in a suitable oxidizing environment to provide an oxide film thereon.

2. The method of inhibiting stress corrosion cracking in austenitic stainless steels subjected to chloride environments which comprises; utilizing an austenitic stainless steel which has an ordinary oxide film on the surface thereof, burnishing the oxide film surface with a piece of solid metal selected from the group consisting of silver and lead in a uniform manner so that substantially all of the steel surface area has been covered by minutemetal particles deposited by the burnishing action in amounts of less than 2 ,ugm. per square centimeter, and wiping the steel surface to remove nonadherent metal particles.

3. The method of inhibiting stress corrosion cracking in austenitic stainless steels subjected to chloride environments which comprises; burnishing the surface of the austenitic stainless steel with a piece of solid metal selected from the group consisting of silver and lead in a uniform manner so that substantially all of the steel surface area has been covered by minute metal particles deposited by the burnishing action in amounts of less than 2 igrn. per square centimeter, wiping the steel surface with a suitable material to remove any nonadherent metal particles, and corroding the steel surface in a suitable oxidizing environment to provide an oxide film thereon.

4. The method of inhibiting stress corrosion cracking in austenitic stainless steels subjected to chloride environments which comprises; utilizing an austenitic stainless steel which has an ordinary oxide film on the surface thereof, and electroplating at least one metal selected from the group consisting of silver, lead and cobalt onto the oxide film in an amount of less than 2 ,ugm. per square centimeter of surface area.

5. The method of inhibiting stress corrosion cracking in austenitic stainless steels subjected to chloride environments which comprises; cleaning the austenitic stainless steel surface with a suitable acid to remove any corrosion products therefrom, electroplating at least one metal selected from the group consisting of silver, lead and cobalt onto the cleaned steel surface in an amount of less than 2 ,ugrn. per square centimeter of surface area, and corroding the steel surface in a suitable oxidizing environmerit to provide an oxide film thereon.

References Cited UNITED STATES PATENTS 2,115,733 5/1938 Krivobok 128 2,215,734 9/1940 Harder 75128 2,267,866 12/1941 Kaye et al. 75-128 RALPH S. KENDALL, Primary Examiner. 

